1. Field of the Invention
This invention relates to a transfer function system for measuring the modulation transfer function of a single optical fiber.
2. Description of the Prior Art
The known optical fiber transfer function measuring systems comprises a generator for producing a first electrical signal, a laser source for converting the first electrical signal into an initial modulated light signal to be delivered to the entrance end of the optical fiber to be measured, an optical detector for converting the outputting light signal from the emergent end of the optical fiber into a second electrical signal and spectrum analyzis means for displaying the spectral lines of the second electrical signal as a function of the frequency.
Two types of transfer function measuring systems have been proposed in the prior art.
A first type of system concerns the swept frequency method. The first electrical signal producing generator is a vobulator which produces a first electrical signal whose frequency varies continually and cyclically. This first electrical signal is transmitted to the laser source which converts it into a carrier light signal modulated by the vobulation frequency. The modulated light signal is transmitted to the entrance end of the optical fiber through an optical lens device. Such a measuring system is disclosed in the contribution No. 208, Study Group XV, of the International Telegraph and Telephone Consultative Committee (C.C.I.T.T.) October 1978, entitled "Considerations on transmission characteristics of measurement methods of optical fibers".
During a calibration phase, i.e. when the laser source is in direct optical coupling with the optical detector, without coupling through the optical fiber to be measured, a response curve is stored in a digital memory of spectrum analyzis means so as to serve as a reference for the ulterior measurement of the transfer function of the optical fiber. This reference response curve is not a constant function independent on the vobulation frequency band because it is dependent on the transfer functions peculiar to the laser source and the optical detector. Consequently, at the time of the measurement phase for which the optical fiber is inserted between the laser source and the optical detector, it is necessary to calculate the difference, relative to the power of the second electrical signal transmitted by the output of the optical detector to the spectrum analysis means, between stored reference response curve and the response curve obtained during the measurement phase, in order to deduce the modulation transfer function proper to the optical fiber. This measurement method does not take into account the optimal conditions for injection of the light signal into the entrance end of the optical fiber to be measured. The injection conditions are relative to the mode coupling of the light signal which is normally balanced in an optical fiber after transmission through it over a great length. Because the radiation pattern of the injected light signal into the entrance end of the optical fiber to be measured is not equal to the balanced radiation pattern in the optical fiber, it means that the measurement according to the above method does not give an exact knowledge of the transfer function proper to the optical fiber.
Moreover, it will be noted that the system according to the vobulation measurement method requires that the vobulator sweeping frequency to be transmitted through an auxiliary electrical conductor to the spectrum analyzis means. This excludes the measurements of an optical fiber having a great length, such as those relative to the laying of an optical multifiber cable on a worksite, where the entrance and emergent ends are not adjoining or in the same room.
The second type of known systems and methods for measuring the transfer function of an optical fiber are based on the pulse response of the optical fiber. In this case, the generator producing the first electrical signal modulating the laser emission consists of an adjustable frequency short pulse generator. The measurement method on the reception side are based on time analyzis, or on the spectral analyzis of a short light pulse delivered by the optical fiber. Such methods and systems are described in communication V. , pages 123 to 134, of the Second European Symposium on Optical Fiber Transmission, September 1975 and concerning spectral analyzis, in the article on pages 43 to 48, Vol. 25, No. 1, of the journal "Optics Communications", April 1978, and also in French Pat. No. 2,296,842.
According to the time analyzis method, the dynamic measurement is reduced and the precision depends on the sharpness of the light pulse transmitted to the entrance end of the optical fiber. This method requires powerful calculation facilities to obtain the relatively imprecise transfer function of the optical fiber.
The low power contained in each spectral line, especially at high frequencies, also reduces the precision of the transfer function measured according to the spectral analyzis method.
On the other hand, certain systems set up according to pulse response measurement method take into account the attainment of a balance state in the mode coupling at the entrance end of the optical fiber. In this connection, a mode balance simulator is interdonnected optically between the laser source and the entrance end of the optical fiber. This simulator can be made by compressing the first 20 centimeters of the optical fiber between some emery cloth and an elastomeric plate which impose some random microcurves on the fiber thereby inducing a strong coupling of the modes. According to another embodiment, the simulator is made with a so-called priming optical fiber which has the same structural features as the ones of the optical fiber to be measured and which is interconnected optically between the laser source and the optical fiber to be measured. The priming optical fiber is very long, about a kilometer, so that the irregular distribution of the propagation modes in front of the laser source at the entrance end of the priming optical fiber becomes progressively homogenous or uniform as one moves away from it. Consequently, a balance state is obtained before the emergent end of the priming optical fiber and corollary at least at the entrance end of the optical fiber to be measured.